This invention relates generally to an improved igniter for a rocket motor having a single-port nozzle and wherein, owing to technical requirements, ignition must be achieved from the aft-end of the motor.
An igniter typically consists of a pyrotechnic charge housed in a container together with a squib or ignition cap. The squib is detonated using an electric current, the energy released by the squib igniting the pyrotechnic charge, which in turn uniformly ignites the entire exposed surface of the propellant disposed within the rocket motor body.
Rockets for use in military operations must be capable of providing reliable performance even though the igniter may be subjected to a wide variety of environmental extremes. The aft or nozzle-end ignition system must ignite the rocket motor reliably, without ignition spikes or hangfires, over a temperature range of approximately -65.degree. F. to 150.degree. F. The pyrotechnic charge must not be allowed to crack or fragment when subjected to vibration or thermal shocks, and the igniter must be installed as an integral part of the rocket yet after ignition produce only a relatively small lightweight ejecta fragments.
Prior to the development of the aft-end ignition system for rocket motors as described in Canadian Patent No. 1,036,419 granted Aug. 15th, 1978 and naming Ellas P. Morris and Fred A. Christie as joint inventors, the great majority of rocket motors were ignited using igniters fixed to the head end of the rocket motor. These particular systems, and the few types of aft-end ignition systems available before that time were incapable of meeting the design requirements mentioned above while at the same time being acceptable for aircraft launching, it being kept in mind that the ejection of large pieces of material during ignition could damage the rocket or the following aircraft or cause unstable combustion and consequently elevated operating pressures. There are also the requirement that the rocket unit be self-contained, of relatively small diameter and relatively inexpensive to construct.
The various requirements outlined above are substantially satisified by the aft-end ignition system set forth in the above Canadian Patent No. 1,036,419. Basically, this system uses a lightweight igniter having a breakable plastic casing which is positioned in the throat of the rocket motor nozzle. The igniter casing is provided with retaining means arranged such that during the detonation of the ignition squib or cap, the igniter is first held in place and then is permitted to move outwardly of the nozzle throat with the retaining means acting on the igniter casing so as to cause the latter to break into fragments with the result being that the igniter totally collapses and is ejected from the rocket motor in relatively small pieces thus avoiding danger to any following aircraft as well as avoiding disturbances in the operation of the rocket motor.
While the aft-end ignition system described in the above Canadian patent has proven to be very successful, one potential problem area remains, this being the provision of an effective weather seal between the igniter body and the nozzle of the rocket motor. It is very important that an effective moisture-proof seal be provided which will prevent moisture from passing between the nozzle throat and the igniter and entering into and dampening the propellant and the ignition charge disposed within the rocket body. Any significant degree of moisture will impair the burning characteristics of the propellant and/or the ignition charge and, in severe cases, will cause the rocket motor to hand fire or fail to ignite altogether.
From all of the methods and means available to provide a weather seal for the rocket motor, two were previously used with some degree of success. The first system involved the use of a rubber diaphragm which was positioned within the rocket nozzle expansion cone with its central portions overlying the rearwardly extending face of the igniter and its side walls overlying the adjacent portions of the nozzle expansion cone. The ignition wires for the squib are passed through a small aperture in the center of the diaphragm. An adhesive material was used to bond the diaphragm to the wall of the nozzle and to the end portions of the igniter.
The second weather seal which was developed involves bonding the end portions of the igniter to a conical foam plug shaped so as to snugly fit into the nozzle expansion cone. The adhesive materials held to secure this form plug in position. A relatively wide passageway was left to accommodate the squib ignition wires. The final sealing was accomplished by the use of a silicone potting compound, which compound completely filled the above-noted passageway as well as covering the entire exposed end of the form plug.
Both of the above-noted weather seal designs had certain limitations. Although the rubber diaphragm arrangement provided an adequate seal when properly bonded, actual production line conditions often introduced wide variations in some important process parameters such as the amount of adhesive applied, the surface treatment of the adherents, and in the manner of application of the adhesive etc. Accordingly, the strength of the diaphragm-to-nozzle bond was often poor and the bond line discontinuous and hence the resulting assembly was prone to leakage.
The silicone potted foam plug arrangement also involves adhesives, treatment of surfaces, etc. and is susceptible to essentially the same problems in actual production line conditions as is the diaphragm arrangement. Moreover, because the potted foam plug is relatively massive, it tends to induce an overpressure in the rocket motor immediately after ignition.